Compositional design of porous beta-Sisub(3)Nsub(4) prepared by pressureless-sintering compositions in the Si-Y-Mg-(Ca)-O-N system

Abstract

Porous β-Si3N4 can be microstructurally engineered to provide similar mechanical properties to dense β-Si3N4, with the additional benefits of reduced mass. However, while flexural strengths exceeding 1 GPa can be achieved by tailoring the microstructure of the material, through alignment of anisotropic β-Si3N4 grains, the porosity levels of such materials are still relatively low (< 15 vol. %). This also requires moderately complex processing techniques to be employed, such as tape-casting and sinter-forging. In the present study, an alternate approach is taken where compositional design is advocated, with the aim of producing a more porous β-Si3N4 structure (ideally 20-40 vol. % porosity) with high grain aspect ratios (i.e. >10:1). This approach makes use of a low volume fraction of multiple sintering aids, where each additive plays one or more roles in the sintering behavior of Si3N4 (i.e. densification aid, transformation aid and/or whisker growth aid). Compositions are based on various ratios of Y2O3:MgO, both with and without CaO additions. Sintering has been conducted in a nitrogen atmosphere (0.1 MPa) between 1400 and 1700°C. The effects of processing parameters (e.g. composition, sintering temperature and time) will be discussed, paying particular attention to microstructure development, including both the densification behavior and α- to β-Si3N4 transformation kinetics.